Erythrocyanin is a red or blue copper-containing protein involved in electron transfer in certain bacteria, such as *Pseudomonas aeruginosa*. It's a type of cupredoxin, characterized by a copper ion cofactor bound to a conserved cysteine-histidine motif within its structure. Erythrocyanin accepts and donates electrons through redox reactions, participating in various metabolic processes, including respiration and photosynthesis.